The concentration of Dissolved Oxygen (D.O) in a river is an important indicator of its water quality. D.O is essential for the survival of aquatic organisms as it is required for their respiration. The reduction in the concentration of D.O indicates the presence of pollutants in the water, which can be harmful to aquatic life.


The zone of degradation is the area of the river where the concentration of pollutants is high, leading to a decrease in the D.O levels. This zone is characterized by the presence of organic matter and pollutants, which undergo decomposition by bacteria and other microorganisms. This decomposition process consumes a significant amount of oxygen, leading to a decrease in D.O levels.


The zone of recovery is the area downstream from the zone of degradation where the concentration of pollutants decreases, and the D.O levels start to recover. In this zone, the decomposition process slows down, and the oxygen demand decreases, allowing the D.O levels to increase.


The zone of active decomposition is the area within the zone of degradation where the decomposition process is at its peak. This zone is characterized by high levels of organic matter and pollutants, resulting in a significant decrease in D.O levels.


The zone of clean water is the area downstream from the zone of recovery where the concentration of pollutants is minimal, and the D.O levels are relatively high. This zone is characterized by good water quality, suitable for supporting aquatic life.

In the given question, the concentration of D.O is reduced to 40%. This indicates that the river is in the zone of active decomposition, where the decomposition process is at its peak and the D.O levels are significantly reduced due to high levels of organic matter and pollutants.